use strict;
use warnings;
use Data::Dumper;

my $ref_a = [
             [ qw/a b c/ ],
             [ qw/d e f/ ],
             [ qw/g h i/ ],
             [ qw/x y z/ ],
             [ qw/3 4 5/ ],
             [ qw/6 7 9/ ],
            ];

my $ref_b = [
             [ qw/j k l/ ],
             [ qw/m n o/ ],
             [ qw/p q r/ ],
            ];

for my $i (0..$#$ref_a) {
    push @{$ref_a->[$i]}, @{$ref_b->[$i]};
}
print Dumper($ref_a);